In late November, Mitt Romney sat for a rare interview, with Bret Baier of Fox News. It didn't go well, and Romney returned to the set to complain to Baier about the questions. Baier went public with the encounter, making Romney seem petty and ridiculous.

From accounts of the episode, it seems that Romney's staff tried to talk him out of going back to harangue Baier; what they didn't do was volunteer to chew out Baier for him.

You need Eric Fehrnstrom for that. Fehrnstrom, who was not with Romney in New York that day, has no qualms about looking petty and ridiculous in Romney's place. It's become something of a professional specialty for him — and it's what makes him absolutely invaluable, and irreplaceable.

That's why I and other longtime Romney watchers shrugged off declarations by some pundits — and even a few Republican notables — that Fehrnstrom would have to be fired after last week's Etch A Sketch gaffe.

Fehrnstrom had handed political manna to his boss's enemies, by declaring in a CNN interview that conservative positions taken during the nomination campaign could be erased "like an Etch A Sketch" before the general election.

Romney didn't so much as publicly rebuke Fehrnstrom. After roughly 24 hours on the bench, the feisty 51-year-old was back on Twitter, mocking the overreaction: "Etch A Sketch stock is up? Psst, I'll mention Mr. Potato Head next. Buy Hasbro."

That Tweet is classic Fehrnstrom — brash, self-assured, funny, fearless, and perfectly designed to grab a media narrative by the horns and yank it in another direction.

Few other political operatives are capable of Fehrnstrom-level ballsy hostility, and that's precisely why Romney can't ever get rid of him. Because that is exactly what the hot-tempered, manipulative, controlling, and combative Romney would do himself, if he didn't have to protect his image as the candidate.

Their dynamic was briefly on display in an episode four years ago, in what had been, until last week, Fehrnstrom's biggest moment in the public spotlight.

It came in a testy exchange between Romney and reporter Glen Johnson, then with Associated Press and now political director for boston.com. Beginning at a press availability, the argument continued afterward, still on camera, with a clearly agitated and argumentative Romney seeking out Johnson. Fehrnstrom saved his boss, stepping in like a coach protecting his player from ejection, berating Johnson while Romney was led away.

Fehrnstrom bullies reporters, insults them, tricks them, plays them off each other, and lies to them outright. I've seen it myself. (I've also seen his pettiness; Fehrnstrom has long blocked me from following him on Twitter.) He does this convincingly, often with righteous anger, even when he is defending what he knows full well are carefully crafted fibs — such as the one Johnson was challenging, of a phony distinction between lobbyists on McCain's campaign and those on Romney's.

Romney needs a guy like Fehrnstrom — and there aren't many like him.

BOSTON FIGHTER

He doesn't fit into any of the city's ethnic stereotypes (his lineage stems from Sweden), but Fehrnstrom is pure pugnacious Boston. Born and raised in the city, now living close by in Brookline, Fehrnstrom studied journalism at Boston University and quickly landed on staff with the Boston Herald. Aggressive, combative, and street-smart, capable of holding grudges or quickly erasing them over a beer, he was a perfect fit for the tabloid and the city.

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